The present invention relates to anion exchange membranes and, more particularly, to anion exchange membranes based on a poly(phenylene) backbone prepared by a Diels-Alder polymerization reaction.
Anion exchange membranes (AEMs) have been used mainly in electrodialysis for the desalination of brackish water and for the production of table salt from sea water. Interest has grown in more demanding applications such as alkaline fuel cells and electrolysis. Hydrogen fueled alkaline fuel cells consisting of a liquid electrolyte (KOH) have demonstrated the ability to generate high power densities. The main advantage of the alkaline media as compared to other low temperature fuel cells is that the kinetics at the cathode oxygen reduction reaction and anode process are much more facile. In addition many non-noble metals and their corresponding oxides are stable in alkaline media. For these reasons platinum based catalysts are not needed and non-noble metal catalysts can be employed. Other technical issues have prevented the alkaline liquid electrolyte fuel cell from being a viable commercial power generator. Many of these issues are associated with the liquid electrolyte. For example, ultra pure oxidants must be used to prevent electrode degradation caused by the formation of carbonate species. Liquid electrolyte cells (and particularly the circulated electrolyte ones) were shown to be complex in design and operation. Over the last decade attempts have been made to develop alkaline anion-exchange membranes and eliminate the issues related to the liquid electrolyte. The main concern with alkaline anion-exchange membranes is the long term stability. Under operation at elevated temperatures membrane degradation occurs by beta-hydrogens undergoing the Hoffmann elimination reaction.
Useful would be a robust alkaline anion exchange membrane immune to such degradation. Unlike cation-exchange membranes where highly durable perfluorinated membranes dominate the market, commercially-available AEMs are typically based on crosslinked polystyrene and are not very stable in alkaline environments. In addition, they are generally blended with other inert polymers and fabric supports that limit their ionic conductivities. Thus there is a need to develop new AEMs that not only have high conductivities and ion selectivities, but that also exhibit excellent chemical stability at high pH values and elevated temperatures.